No abstract

Tacca, a genus of tropical herbs, possesses near black flowers, conspicuous involucral bracts and whisker-like filiform bracteoles. These features have been assumed to function as a "deceit syndrome" in which reproductive structures resemble decaying organic material attracting flies that facilitate cross-pollination (sapromyiophily). We investigated pollination and mating in Tacca chantrieri populations from SW China to evaluate this assumption. Contrary to this expectation, populations were highly selfing. Pollinator visitation was infrequent and bagged flowers set abundant seed. Pollen loads on stigmas indicated autonomous self-pollination, some of which occurred prior to flower opening. The seed set of inflorescences with bracts and bracteoles removed was not significantly different from unmanipulated inflorescences, suggesting that these structures play a limited role in pollinator attraction, at least at our study sites. Pollen : ovule ratios averaged 49, a value expected in a highly selfing species. Selfing rates estimated in four populations using allozyme markers averaged 0.86 (range 0.76-0.94), thus corroborating this inference. Our results indicate that despite considerable investment in extravagant display, populations of T. chantrieri are highly selfing. We propose several hypotheses to resolve this paradox and argue that future studies of pollination syndromes would benefit by investigation of both pollination and mating biology.

Orchid conservation efforts, using seeds and species-specific fungi that support seed germination, require the isolation, identification, and germination enhancement testing of symbiotic fungi. However, few studies have focused on developing such techniques for the epiphytes that constitute the majority of orchids. In this study, conducted in Xishuangbanna Tropical Botanical Garden, Yunnan, China, we used seeds of Dendrobium aphyllum, a locally endangered and medicinally valuable epiphytic orchid, to attract germination promoting fungi. Of the two fungi isolated from seed baiting, Tulasnella spp. and Trichoderma spp., Tulasnella, enhanced seed germination by 13.6 %, protocorm formation by 85.7 %, and seedling development by 45.2 % (all P < 0.0001). Epulorhiza, another seed germination promoting fungi isolated from Cymbidium mannii, also enhanced seed germination (6.5 %; P < 0.05) and protocorm formation (20.3 %; P < 0.0001), but Trichoderma suppressed seed germination by 26.4 % (P < 0.0001). Tulasnella was the only treatment that produced seedlings. Light increased seed imbibition, protocorm formation, and two-leaved seed development of Tulasnella inoculated seeds (P < 0.0001). Because the germination stage success was not dependent on fungi, we recommend that Tulasnella be introduced for facilitating D. aphyllum seed germination at the protocorm formation stage and that light be provided for increasing germination as well as further seedling development. Our findings suggest that in situ seed baiting can be used to isolate seed germination-enhancing fungi for the development of seedling production for conservation and reintroduction efforts of epiphytic orchids such as D. aphyllum.

Background As in most land plants, the roots of orchids (Orchidaceae) associate with soil fungi. Recent studies have highlighted the diversity of fungal partners involved, mostly within Basidiomycotas. The association with a polyphyletic group of fungi collectively called rhizoctonias (Ceratobasidiaceae, Tulasnellaceae and Serendipitaceae) is the most frequent. Yet, several orchid species target other fungal taxa that differ from rhizoctonias by their phylogenetic position and/or ecological traits related to their nutrition out of the orchid roots (e.g., soil saprobic or ectomycorrhizal fungi). We offer an evolutionary framework for these symbiotic associations. Scope Our view is based on the ‘Waiting Room Hypothesis’, an evolutionary scenario stating that mycorrhizal fungi of the land flora were recruited from ancestors that initially colonized roots as endophytes. Endophytes biotrophically colonize tissues in a diffuse way, contrasting with mycorrhizae by the absence of morphological differentiation and of contribution to the plant’s nutrition. The association with rhizoctonias is likely the ancestral symbiosis that persists in most extant orchids, while during orchid evolution numerous secondary transitions occurred to other fungal taxa. We suggest that both the rhizoctonia partners and the secondarily acquired ones are from fungal taxa that have broad endophytic ability, as exemplified in non-orchid roots. We review evidence that endophytism in non-orchid plants is the current ecology of many rhizoctonias, which suggests that their ancestors may have been endophytic in orchid ancestors. This also applies to the non-rhizoctonia fungi that were secondarily recruited by several orchid lineages as mycorrhizal partners. Indeed, from our review of the published literature, they are often detected, likely as endophytes, in extant rhizoctonia-associated orchids. Conclusion The orchid family offers one of the best documented examples of the ‘Waiting Room Hypothesis’: their mycorrhizal symbioses support the idea that extant mycorrhizal fungi have been recruited among endophytic fungi that colonized orchids ancestors.